Digital radio broadcasting technology delivers digital audio and data services to mobile, portable, and fixed receivers. One type of digital radio broadcasting, referred to as in-band on-channel (IBOC) digital audio broadcasting (DAB), uses terrestrial transmitters in the existing Medium Frequency (MF) and Very High Frequency (VHF) radio bands. HD Radio™ technology, developed by iBiquity Digital Corporation, is one example of an IBOC implementation for digital radio broadcasting and reception.
IBOC technology can provide digital quality audio, superior to existing analog broadcasting formats. Because each IBOC signal is transmitted within the spectral mask of an existing AM or FM channel allocation, it requires no new spectral allocations. IBOC promotes economy of spectrum while enabling broadcasters to supply digital quality audio to the present base of listeners.
The National Radio Systems Committee, a standard-setting organization sponsored by the National Association of Broadcasters and the Consumer Electronics Association, adopted an IBOC standard, designated NRSC-5, in September 2005. NRSC-5, the disclosure of which is incorporated herein by reference, sets forth the requirements for broadcasting digital audio and ancillary data over AM and FM broadcast channels. The standard and its reference documents contain detailed explanations of the RF/transmission subsystem and the transport and service multiplex subsystems. Copies of the standard can be obtained from the NRSC at http://www.nrscstandards.org/standards.asp. iBiquity's HD Radio technology is an implementation of the NRSC-5 IBOC standard. Further information regarding HD Radio technology can be found at www.hdradio.com and www.ibiquity.com.
IBOC signals can be transmitted in a hybrid format including an analog modulated carrier in combination with a plurality of digitally modulated carriers or in an all-digital format wherein the analog modulated carrier is not used. Using the hybrid mode, broadcasters may continue to transmit analog AM and FM simultaneously with higher-quality and more robust digital signals, allowing themselves and their listeners to convert from analog-to-digital radio while maintaining their current frequency allocations.
Both AM and FM In-Band On-Channel (IBOC) hybrid broadcasting systems utilize a composite signal including an analog modulated carrier and a plurality of digitally modulated subcarriers. Program content (e.g., audio) can be redundantly transmitted on the analog modulated carrier and the digitally modulated subcarriers. The analog audio is delayed at the transmitter by a diversity delay.
In the absence of the digital audio signal (for example, when the channel is initially tuned) the analog AM or FM backup audio signal is fed to the audio output. When the digital audio signal becomes available, a blend function smoothly attenuates and eventually replaces the analog backup signal with the digital audio signal while blending in the digital audio signal such that the transition preserves some continuity of the audio program. Similar blending occurs during channel outages which corrupt the digital signal. In this case the analog signal is gradually blended into the output audio signal by attenuating the digital signal such that the audio is fully blended to analog when the digital corruption appears at the audio output. Corruption of the digital audio signal can be detected during the diversity delay time through cyclic redundancy check (CRC) error detection means, or other digital detection means in the audio decoder or receiver.
The concept of blending between the digital audio signal of an IBOC system and the analog audio signal has been previously described in, for example, U.S. Pat. Nos. 7,546,088; 6,178,317; 6,590,944; 6,735,257; 6,901,242; and 8,180,470, the disclosures of which are hereby incorporated by reference. The diversity delay and blend allow the receiver to fill in the digital audio gaps with analog audio when digital outages occur. The diversity delay ensures that the audio output has a reasonable quality when brief outages occur in a mobile environment (for example, when a mobile receiver passes under a bridge). This is because the time diversity causes the outages to affect different segments of the audio program for the digital and analog signals.
In the receiver, the analog and digital pathways may be separately, and thus asynchronously, processed. In a software implementation, for example, analog and digital demodulation processes may be treated as separate tasks using different software threads. Subsequent blending of the analog and digital signals requires that the signals be aligned in time before they are blended.
Both FM and AM Hybrid In-Band On-Channel (IBOC) HD Radio™ receivers require an audio blend function for the purposes of blending to the FM or AM analog backup signal when the digital signal is unavailable. The maximum blend transition time is limited by the diversity delay and receiver decoding times, and is typically less than one second. Frequent blends can sometimes degrade the listening experience when the audio differences between the digital and analog are significant.
Blending will typically occur at the edge of digital coverage and at other locations within the coverage contour where the digital waveform is corrupted. When a short outage does occur, such as traveling under a bridge, the loss of digital audio is replaced by an analog signal. When blending occurs, it is important that the content on the analog audio and digital audio channels are aligned in both time and level (i.e., loudness) to ensure that the transition is barely noticed by the listener. Optimally, the listener will notice little other than possible inherent quality differences in analog and digital audio at these blend points. However, if the broadcast station does not have the analog and digital audio signals aligned, then the result could be a harsh sounding transition between digital and analog audio. The misalignment may occur because of audio processing differences between the analog audio and digital audio paths at the broadcast facility. Furthermore the analog and digital signals are typically generated with two separate signal generation paths before combining for output. The use of different analog processing techniques and different signal generation methods makes the alignment of these two signals nontrivial. The blending should be smooth and continuous, which can happen only if the analog and digital audio are aligned in both time and level.
It would be desirable to process a digital radio signal in a manner that allows blending of the digital and analog components without an abrupt change in loudness of the audio output.